Amplifying arrangement



June 21, 1938.

A. VAN- DE RlET 2,120,995

AMPLIFYING ARRANGEMENT Filed Jan. 25, 1933 INVENTOR ABFaAHAM VAN DE RIETATTORNEY Patented June 21, 1938 VUNITED STATES PATENT oFFics Abraham vande Biet, Eindhoven, Netherlands, ,assignor to Radio Corporation ofAmerica a corporation of Delaware "Application January 23, 1983,-SerialNo.- 652,996 I In the Netherlands April 26, 1932 a Claims.

7 direct current energy delivered is reduced. A re-' duction of thedirect current energy in the output circuit can be obtained at a givenanode voltage by the use of a negative grid bias. The value of this gridbias is, however, limited by the condition that the valve should alwaysoperate on a linear part of the anode current-grid voltagecharacteristic curve. That is to say, the algebraic sum of the negativegrid bias and the maximum alternating voltage to be amplified should notbe so large that the valve operates on a curved part of thecharacteristic.

In well known circuit arrangements, generally, constantvoltages aresupplied to the anode and grid of the amplifying valve. The value ofthese voltages is dependent upon the conditions that a given anodedissipation should not be exceeded, and that a certain maximum voltageshouldbe susceptible to a. linear amplification.

Independently of the value of the alternating voltage to be amplified ata given moment, a

constantdirect current energy'is delivered by a valve adjusted in themanner .of said well'known which the direct current energy delivered isnot constant, and in which this energy is smallerthan in the. case abovedescribed so that a higher alternating current energy can be admittedfor a given valve. The amplifying arrangement, according to theinvention, is characterized by a grid bias which is so variable with thesignal voltage to be amplified that when the signal voltage decreasesthe grid bias becomes more negative.

Thus, in thecase of feeble signals; a high, and, in the case of morepowerful signals, a lower negative grid bias is applied to the amplifiervalve. In any case this grid biasv should besuch that the algebraic sumof the bias and of the voltage to be amplified is smaller than a voltageby which the valve would be adjusted to a curved partof thecharacteristic curve. Preferably, the adjustment is such that the. sumhereinbefore indicatedof the grid voltages is substantially constant,and is so large as to prevent :thevalve 'fromiop'erating onacurved partof the characteristic,

(Cl. 179L171) This arrangement ensures that in the case of feeblesignals, also, the direct current energy delivered is smaller than inthe case of powerful signals. Thus, the total direct current energydelivered decreases when receiving weak signals, whereas, thealternating current energy delivered remains unvaried so that the totalquantity of energy delivered is reduced, and the tube is not fullyloaded. Consequently, the primarily impressed signal voltage can beincreased. This results in an increase of anode direct and alternatingcurrent energy until the admissible load of the tube is reached again.

The circuit arrangement according to the invention is particularlyadvantageous for valves having to deliver a high power; consequently,for

example, for the last valve, in an amplifying circuit arrangement.

The alternating current energy delivered by a given type of amplifyingvalve is increased by the use of the invention.

In order that the invention may be clearly understood and readilycarried into effect two embodiments thereof will now be described morefully with reference to the accompanying drawing, in which:

,Fig. 1 shows thelast amplifying stage of a battery receiver embodyingthe invention;

Fig. 2 shows a modification employing power amplifier supplied withalternating current.

Referring to Fig. 1, I designates a thermionic valve illustrated as ascreen grid valve. Its input circuit includes an impedance 2 which is,forexample, the secondary of a transformer to which thevoltages to beamplified are supplied. The output circuit of the valve includesanoutput impedance 9; for example, the primary of a trans former towhich a loudspeaker may be connected. The screen grid is connected to apoint of the anode voltage battery (not shown to preserve simplicity ofdisclosure).

The valves used may comprise directly, or indirectly, heated cathodeswhich are supplied with direct, or alternating, current. Generally, itis desirable to withdraw the voltages to be rectified fromthe outputcircuit of the amplifier, as the voltages which occur in the input[circuit are often too small to ensure a proper control, and moreover, aconnection of the rectifier in parallel with the input. circuit resultsin a rather great decrease of the amplification. Generally, therefore, apart of the output voltage will be rectified, and in this .caseisaidpart should be so large that-the grid voltage of the amplifying valve isconstantly as much as possible negative but always so that no operationensues in a curved part.

The diagram shown in Fig. 1 represents the last stage of a batteryamplifier and can be used, for example, in a portable set. Such a deviceis always supplied with direct current. In this case it is importantthat the cathodes of all the valves should be connected to the samesource of voltage. This condition is satisfied in the circuitarrangement illustrated. A triode I2 which is connected as a griddetector is used as the rectifier. The anode circuit of this valveincludes a high resistance I3. When no signal is impressed on the inputcircuit 2 of valve I, the anode current of valve 7 I2 is a maximum.

The potential drop across the resistance I3 is with the voltage ofbattery I! is supplied to the grid of valve I by means of apotentiometer M, I5, H3; Thus, the negative grid voltage of this valveis rendered a maximum, but the grid voltage is not negative enough tooperate the tube on the curved portion of the characteristic. Now, if analternating voltage is supplied to the input circuit of valve I, thisresults in that the voltage at the input circuit of the detector islowered, the current passing through resistance I3 decreases, and thevoltage at the plate of valve I2 consequently increases. Thus, the gridvoltage of valve I becomes less negative. The grid bias is smoothed bymeans of condensers I8 and Ill. The condensers 20 and 2| constitute acapacitative potentiometer on the output circuit by which a desired partof the voltage is supplied to the detector valve. The condenser 20serves simultaneously as a grid condenser for this detector valve.

If a final valve of very great power is used,

preferably an arrangement as shown in Fig. 2 may be utilized. In thisarrangement use is made of valves supplied with alternating current. Thegrid bias for the amplifying valve is delivered by two rectifierspreferably housed in one valve and having one common cathode. Therectifier formed by the cathode 23 and the anode 24 is connected to awinding 39 of the public supply transformer 3| and consequently deliversa constant rectified voltage. The rectifier formed by the cathode 23 andthe anode 25 is connected to a part of the output impedance 9 of valveIso that this rectifier produces voltages the'value of which depends onthe signal voltage. The two rectified voltages are smoothed in knownmanner, and are supplied in opposite directions to the input circuit ofthe amplifying valve so that the bias of this valve varies in thedesired manner with the signalintensity. The current from supply source3| is rectified, and develops across resistor 40 a direct currentvoltage. Since the signal grid is connected to the anode side ofresistor 40, the signal grid is negatively biased to a constant value;as explained previously this value is not great enough to operate thetube I on the curved portion of the characteristic- A portion of thesignal energy is rectified by diode 2523; resistor H having developedthereacross a direct current voltage proportional in magnitude to'thesignal intensity. Since the resistor 4| is connected in series betweenthe cathode of tube I and resistor 40, and the positive end of thelatter is connected tothe positive terminal of resistor 4|, the voltagesacross bothresistors oppose each other. I-Ience, with increasing signalintensity the negative bias of grid I is reduced by the positive voltageacross resistor 4|. Each of resistors 4|] and 4| is shunted by acondenser of low impedance to signal currents. Resistor Ell functions asa high impedance to signal currents; condenser 5| is a path of lowimpedance to current from source 3I.

While I have indicated anddescribed several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

" What is claimed is:

1. In combination with an audio frequency amplifier, an audio frequencysignal input circuit coupled to the amplifier input electrodes, anoutput circuit coupled to the output electrodes of the amplifier, asource of alternating current potential, a diode rectifier coupled tosaid. source,

'a direct current connection between said input circuit and the diodeanode whereby the amplifier signal grid is maintained at a negativedirect current potential with respect to the cathode, and means,responsive to variations in the amplifier output amplitude, for varyingsaid signal grid bias.

2. In combination with an audio frequency amplifier, an audio frequencysignal input circuit coupled to the amplifier input electrodes, anoutput circuit coupled to the output electrodes of the amplifier, asource of alternating current potential, a diode rectifier coupled tosaid source, and a direct current connection between said input circuitand the diode anode whereby the amplifier signal grid is maintained at anegative direct current potential with respect to the oathode, a secondanode in the envelope of said diode, and a path of low impedance to theaudio output of said amplifier connected between the amplifier outputcircuit and said second anode.

3. In combination with an audio frequency amplifier, an audio frequencysignal input circuit coupled to the amplifier input electrodes, anoutput circuit coupled to the output electrodes of the amplifier, asource of alternating current potential, a diode rectifier coupled tosaid source, and a direct current connection between said' input circuitand the diode anode whereby the amplifier signal grid is maintainedoutput circuit and the second anode, said diode cathode and second anodebeing connected together toprovidea diode rectifier for developing abiasing direct current voltage from amplified signal currents whichbiases said amplifier in a direction to increase the amplifier platecurrent flow.

4. In combination with a source of audio frequency signals, an audiofrequency signal amplifier, a potential source for applying a c0nstantnegative bias to the amplifier grid sufficient in magnitude to adjusttheamplifier operation to a predetermined point on the linear portion ofthe amplifier grid potential-plate current characteristic, a rectifiertube coupled to the amplifier output circuit to rectify audio sig nails,a resistor connected in the space current path of said rectifierfor'developing an amplifier grid biasing voltagewhich varies inmagnitude with the amplitude of the audio signals impressed on therectifier, means for impressing said varying bias voltage on theamplifier grid in a sense to decrease said negative bias, and themagnitude of said Varying biasing voltage being maintained Within suchlimits as to operate the amplifier over the linear portion solely of itssaid characteristic.

5. In combination with an electric wave amplifier tube, a source ofwaves, to be amplified. coupled between the wave input grid and cathodeof said tube, said cathode being at a relatively fixed potential, asource of alternating current energy, a rectifier circuit, including animpedance, coupled to said alternating current source, said impedancebeing connected in series between the said grid and cathode of saidamplifier tube and having a magnitude such that the direct currentvoltage developed thereacross, by rectification of said alternatingcurrent, is sufiicient to maintain said control grid at a predeterminednegative direct current potential with respect to the cathode, a secondrectifier circuit, including an impedance, arranged to have wavesamplified by said amplifier impressed thereupon from the amplifieroutput circuit, said impedance being connected between the control gridand cathode of said amplifier tube and having a magnitude such that thedirect current voltage developed thereacross, by rectification of saidamplified waves, biases said control grid in a positive direction withrespect to said cathode.

ABRAHAM VAN DE RIET.

